1. FIeld of the Invention
This invention relates to the removal of tritium, the radioactive isotope of hydrogen, and/or compounds of tritium, from gas streams comprising the same. More especially, the invention relates to the removal of radioactive tritium and compounds thereof from gas streams comprised of same to concentration levels of parts per billion and even parts per trillion by volume.
2. Description of the Prior Art
Tritium is of course an unstable radioactive isotope having a nucleus consisting of a proton with two neutrons. Believed to be formed in nature by cosmic radiation in the upper atmosphere, natural tritium is present in very minute quantities [one tritium atom per 10.sup.18 protium atoms] in atmospheric hydrogen and rainwater. The natural atmospheric tritium inventory has been increased by thermonuclear weapons testing. Artificial and commercial tritium occur by various other methods. For example, formation of natural tritium is believed to result from primary cosmic-ray reactions or by collisions of fast neutrons, protons, and deuterons in cosmic radiation areas of the upper atmosphere. Artificial tritium can be produced in light and heavy water reactors, cyclotrons, cloud chambers, and the like, by, inter alia: thermal-neotron irradiation of deuterium, .sup.3 He and .sup.6 Li in reactor coolants; fission; bombardment of boron, fluorine, and other elements with deuterium; and bombardment of beryllium with deuterons. Tritium is commercially produced via irradiation of enriched lithium or lithium fluoride with subsequent recovery by acid treatment or palladium diffusion techniques.
Tritium itself probably presents a relatively low hazard. It is a low beta emitter, and no special shielding therefor is required. However, it is often encountered in large doses and should be respected accordingly. Moreover, tritium tends to oxidize to tritiated water in which form it is easily absorbed through the skin and lungs. It is therefore a definite hazard and ingestion and inhalation of tritiated vapors should be avoided. While gaseous tritium has a low solubility in body fluids and is about 10.sup.-3 times less toxic than tritiated water, certain tritiated compounds may concentrate in various organs resulting in localized high dosages. The biological half-life for tritium can be defined as the time taken for one-half the assimilated activity to be excreted from the body following a single dose of tritium. The typical biological half-life value for tritiated water ranges from between 9 to 14 days. This value can be decreased by increasing fluid intake.
Thus, in the light of the aforesaid toxicity and the various direct and indirect hazards associated with exposure to the tritium molecule, certain restrictions have been placed upon tolerable exposures to tritium and/or any composition or compound thereof. Compare, inter alia, the effluent limitations imposed upon users in Sections 20 and 50 of Title 10 of the Code of Federal Regulations. Further, various studies have also been performed to develop containment systems to protect against the accidental release of tritium from a research and development operation, or from a reactor fuel reprocessing facility, or from other environments. A need nevertheless exists for additional means for the removal of tritium from a given gaseous environment or gas stream, or from ambient following an accidental release of the tritiated species thereto.